Produced fluids from many offshore hydrocarbon reservoirs contain significant amounts of carbon dioxide and other gases. For example, some fields offshore Brazil, Thailand, Indonesia and others may produce associated gas along with oil that may contain 30-40 vol % CO2 or more. Consistent with regulations and laws, the oil from wells cannot be produced without proper disposal of associated gases. Pipelines to bring this gas to a nearby market may be prohibitively expensive due to the long distances required. Two current methods for disposal of associated gases are either to flare this gas and vent the flue gas to the atmosphere, or to add gas reinjection equipment on to vessel or platform on which the produced fluids are processed. The gas reinjection equipment reinjects the entire gas stream back into a subterranean reservoir. Besides losing the opportunity to monetize the associated gas along with the oil, these conventional approaches have a number of problems.
Increasingly, flaring of the associated gas stream is being banned due to environmental concerns. Second, in the case of reinjection of the associated gas, extensive compression equipment is needed, as the entire gas stream (hydrocarbons and the CO2) need to be injected. Because the associated gas contains high percentages of light hydrocarbon gases (C1-C5), power to compress the associated gas to reach a dense phase or subcritical phase is much higher, as compared to injection of relatively pure CO2. This means that the associated gases would have to be injected at a much higher pressure, which increases the cost and potential subsurface risks.
FIG. 1 illustrates an exemplary prior art system in which produced well fluids 1 from an underwater hydrocarbon production zone (C) is sent to the topsides of a FPSO (floating production, storage and offloading) vessel located in a body of water (B). The produced well fluids 1 are sent to a production separator (E) which separates out by density differences an associated gas stream (4), an oil stream (2), and a produced water stream (3). After suitable water treatment, the produced water (3) is often discharged back into the body of water (B) or reinjected back into the hydrocarbon reservoir (C) or other subterranean reservoir. The oil product of oil stream 2 is often stabilized, and then stored into the hull of the FPSO (A). Stabilized refers to the removal of light gases such C1-C5 gases leaving an oil product with low vapor pressure. Since there is often no available pipeline to export the associated gas stream from the FPSO to a nearby market, the gas stream (4) is either sent to a flare (F) and burned to produce a flue gas (6) or to gas injection compressors (G). Reinjected gas (8) is sent to a gas injection zone (D) that may be located underneath the hydrocarbon production zone (C).
Suggestions have been to optimize the separation of CO2 from associated gases. For example U.S. patent application Ser. No. 12/361,961, entitled Process for Upgrading Natural Gas with Improved Management of Gas, suggests using crosslinked polymer membranes to effectively separate CO2 from natural gas. The CO2 stream may be reinjected into subterranean formations and the natural gas used such as for creating Fischer-Tropsch products. The contents of this application are hereby incorporated by reference in its entirety.
U.S. patent application Ser. No. 12/974,337, entitled Process and System for Blending Synthetic and Natural Crude Oils Derived from Offshore Produced Fluids, discloses that associated gas are separated into CO2 enriched permeate stream and a CO2 depleted gas product stream. The gas product stream is converted into synthesis gas (hydrogen (H2) and carbon monoxide (CO)) by gas reformers and then the synthesis gas is converted by contacting hybrid catalysts in a gas conversion reactor to produce an effluent containing water, gas and hydrocarbons which, preferably, are general free of wax products (C21+) at ambient temperatures and conditions. The wax free product or synthetic crude oil may then be stabilized by removing light gases and blended with the natural crude oil and stored aboard a production platform or vessel. Wax typically causes numerous problems in Fischer-Tropsch operations. Accordingly, wax is often required to be cracked by way of hydrocracker unit back into liquid products at ambient conditions. However, with weight and space being a premium on an offshore platform, producing a generally wax free hydrocarbon liquid or synthetic crude oil does away with the need for such a hydrocracking unit. The contents of this application are hereby incorporated by reference in its entirety.
There are opportunities for improvements in such systems that use polymer membranes to separate CO2 gas from product gases of associated gas and then use the product gas to produce synthetic crude oil suitable for storage or transport on a vessel. The present disclosure describes such improvements.